Paperboard is used in various packaging applications. For example, aseptic liquid packaging paperboard is used for packaging beverage cartons, boxes and the like. Therefore, customers often prefer paperboard having a generally smooth surface with few imperfections to facilitate the printing of high quality text and graphics, thereby increasing the visual appeal of products packaged in paperboard.
Conventionally, paperboard smoothness is achieved by a wet stack calendering process in which the paperboard is rewetted and passed through a calendering device having two or more hard rolls. The wet stack calendering process smoothes the paperboard by compressing the fiber network to reduce the pits and crevices in raw stock paperboard (see FIG. 1). The result is a smooth paperboard with reduced board thickness and bulk and, therefore, reduced stiffness. However, stiffness is an important requirement for many paperboard applications, such as aseptic liquid packaging paperboard. Therefore, preparing a smooth yet stiff paperboard using the conventional wet stack calendering process requires increasing the basis weight of the paperboard, thereby substantially increasing the raw material cost.
Alternatively, manufacturers have attempted to smooth the surface of paperboard by coating the entire surface of the paperboard with a basecoat comprised of various pigments, such as clay, calcium carbonate, TiO.sub.2 and the like, then overcoating this base with a second and sometimes even a third coating, which is generally referred to as a topcoat. It was discovered that high quantities of relatively fine pigment particles applied to the surface of paperboard provided a more smooth surface without the need for wet stack calendering, thereby maintaining bulk. For example, as shown in FIG. 2, it was discovered that relatively high quantities (e.g., 10.6 pounds per 3000 ft.sup.2 or more) of relatively fine ground calcium carbonate, such as CARBITAL®95 (Imerys Pigments, Inc. of Roswell, Ga.), applied to the rough surface of paperboard provided the greatest smoothness. Indeed, it has been understood that the more pigment applied to the surface of the paperboard the better the resulting smoothness. However, the use of relatively high quantities of pigments substantially increases the cost of preparing smooth and highly printable paperboard.
Accordingly, there is a need for a basecoat and associated paperboard structure that maintains paperboard bulk and provides the desired smoothness for high quality printing, while reducing manufacturing cost.